It happened across from Contra Costa College in San Pablo.
Contra Costa Battalion Chief Charles Thomas says that the building was boarded up, and there was heavy smoke and flames when fire crews arrived.
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The crews were able to make an exterior attack to prevent spreading and have since contained the fire. No injuries have been reported.
Thomas says there may have been several unhoused people inside the building.
It is still unconfirmed what caused the fire, but Thomas says officials believe it may have started from a warming fire from someone inside.